Natural rubber (cis-1,4-polyisoprene) is produced in about 2000 plant species (usually as a constituent of plant latex) with varying degrees of quality and quantity. Several well-studied examples of rubber-producing plants include:                1. Indian laurel (Ficus elastica), a well-known household plant that produces a rubber-containing latex.        2. Trees of the Sapotacae family (Palaquim gutta and P. oblongifolia), located in the Malaysian peninsula and responsible for gutta percha latex (a viscous, grayish latex that exudes slowly from cuts in the bark and rapidly turns brown after exposure to the air).        3. The tropical American tree Mimusops balata, which produces Balata latex as white or reddish exudates.        4. The tropical American saprodilla tree Archras zapote, which produces Chicle        5. The Central American tree Castilla elastica, which produces caucho negro rubber.        6. The Brazilian species, Manihot glazovii, which produces ceara rubber.        7. The dandelion species kok-saghyz (Taraxacum kok-saghyz; from Kazakhstan) and krim-saghyz (T. megalorhizon; found in the Crimea and throughout the Mediterranean region), which produce a high-quality rubber in their roots.        8. The non-latex producing American desert shrub guayule (Parthenium argentatum), in which rubber is produced seasonally within parenchymatous cells of the stem and root, and its isolation requires harvesting of the plant and maceration of the tissue.The natural rubbers produced by each of these species differ in one or more of their properties. In particular, differences in molecular weight and molecular weight distribution have been observed in natural rubbers depending on their plant origin (Backhaus, R. A. Israel Journal of Botany 34: 283–293 (1985)).        
Natural rubber, despite the development of many synthetic polymer alternatives, remains a high-volume commodity material based on its superior properties of elasticity, resilience, and resistance to high temperature. Currently, some 6,810,000 tons of natural rubber are produced annually. Despite this abundance, latex tapped from the tree Hevea brasiliensis is today the only significant commercial source of natural rubber and it is expected that global demand will soon be greater than supplies. Thus, there is significant interest in studying rubber biosynthesis and the differences between rubber produced by Hevea to other natural rubbers, in order to develop alternative rubbersources. In particular, it would be useful to industry to have available rubbers with different molecular weight averages (higher and lower than Hevea rubber) and distributions. For example, rubbers with molecular weights lower than those obtained from H. brasiliensis may have distinct advantages over the Hevea material in certain applications due to their ease of processing (Nor, H. M., and Ebdon, J. R. Progress in Polymer Sci. 23: 143–177 (1998); Meeker, T. Low Molecular Weight Polyisoprenes Offer Versatility In Bonding Techniques. Adhesives Age; pp. 23–26 (July 1998)). Although the molecular weights of rubbers synthesized in in vitro experiments with isolated, enzymatically-active rubber particles are highly influenced by the concentrations of initiator allylic diphosphate and isopentenyl diphosphate (IPP), the intrinsic properties of the cis-prenyltransferases themselves also play a role in determining the size of the rubber molecules they produce (Cornish, K. Phytochemistry 57: 1123–1134 (2001)).
Cis-prenyltransferases are a family of enzymes that are responsible for synthesizing natural rubbers, by catalyzing the sequential addition of C5 units (in the form of isopentenyl pyrophosphate (IPP)) to an initiator molecule in head-to-tail condensation reactions. The initiator molecules themselves are derived from isoprene units through the action of distinct prenyltransferases. These initiators are allylic terpenoid diphosphates such as dimethylallyldiphosphate (DMAPP; C5), geranyl diphosphate (GPP; C10), farnesyl diphosphate (FPP; C15), and geranylgeranyl diphosphate (GGPP; C20). Genes encoding the enzymes which synthesize these allylic terpenoid diphosphates have been cloned from a number of organisms, including plants, and all of these genes encode polypeptides with conserved regions of homology (McGarvey et al., Plant Cell 7:1015–1026 (1995); Chappell, J., Annu. Rev. Plant Physiol. Plant Mol. Biol. 46:521–547 (1995)). All of these gene products condense isoprene units in the trans- configuration. Prenyltransferases that condense isoprene units in a cis-configuration have only recently been identified in microbes and plants. Most notable to the present disclosure herein is the discovery of cis-prenyltransferase gene products in latex of the rubber-producing species Hevea brasiliensis (WO01/21650; GenBank Accession Numbers AY124934, AY124474, AY124473, AY124472, AY124471, AY124470, AY124469, AY124468, AY124467, AY124466, AY124465, AY124464; see also AB061236 and AB074307).
In the present disclosure, the problem to be solved therefore is to identify new plant cis-prenyltransferase genes. These genes will have utility in modification of the properties of natural rubbers obtained from plants. Applicants have solved the stated problem by identifying plant genes encoding cis-prenyltransferases from rubber-producing russian dandelion and sunflower species (both of which produce natural rubbers with different properties than those obtained from H. brasiliensis). Additionally, Applicants have discovered diagnostic features within the gene sequences of cis-prenyltransferases from rubber-producing species.